Decline Rate Types (2024)

There are four types of decline rate that can be specified:

• Annual Effective % (Secant)*
• Annual Nominal %
• Monthly Effective % (Secant)*
• Monthly Nominal %

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*For the Annual and Monthly Effective decline rate types, Enersight uses the Secant method by default but there is a User Preference to use the Tangent method.

Effective Declines refer to the percent decline over the first time period. For example a Monthly Effective decline of 20% means that the production declines by 20% over the first month.For a hyperbolic curve however, this is the effective rate at time zero (tangentially to the curve), so the production at the end of the period will not be the initial minus the % effective.
Nominal Declines refer to the instantaneous decline rate at the start of the decline segment. This is the decline rate that is entered into the decline equations, for example an exponential decline with a 20% Annual Nominal decline is described by the equation q(t) = qi * e-0.2t, where t is time in years.

Hyperbolic Decline

q(t) = qi / [Di*b*(t-0.5) +1]^(1/b)

Where:qi= initial rate
Di = Nominal decline rate
b = hyperbolic exponent
t = time in days (note the ½ day rule)
q(t) = average rate for day t

Enersight will convert between the four decline types when the decline rate type is toggled on the Decline tab by clicking on the link above the decline parameters.